use gix::bstr::ByteSlice as _;
use gix::objs::tree::EntryKind;
use gix::{Commit, ObjectId};
use line_numbers::{LineNumber, SingleLineSpan};
use rustc_hash::FxHashSet;
use tracing::info;

/// Blobs larger than this are treated as binary (reported by size only, not diffed),
/// overriding gix's `core.bigFileThreshold` default of 512 MiB. Scoped to diffing only.
const LARGE_FILE_THRESHOLD_BYTES: u64 = 1024 * 1024;

#[derive(Debug)]
pub struct FileContent {
    pub(crate) path: String,
    pub(crate) oid: String,
    pub(crate) size: usize,
    pub(crate) is_binary: bool,
    /// True when this side is a submodule (gitlink); `oid` is then a commit id pointing into
    /// the submodule's repository, not a blob in this one.
    pub(crate) is_submodule: bool,
}

/// A hunk represents a series of modified lines that are displayed
/// together.
#[derive(Debug, Clone)]
pub struct Hunk {
    /// The LHS line numbers that contain novel content.
    pub(crate) novel_lhs: FxHashSet<LineNumber>,
    /// The RHS line numbers that contain novel content.
    pub(crate) novel_rhs: FxHashSet<LineNumber>,
    /// Line pairs that contain modified lines. This does not include
    /// padding, so at least one of the two lines has novel content.
    pub(crate) lines: Vec<(Option<LineNumber>, Option<LineNumber>)>,
}

/// A matched token (an atom, a delimiter, or a comment word).
#[derive(PartialEq, Eq, Debug, Clone)]
enum MatchKind {
    // TBD
}

#[derive(Debug, Clone, PartialEq, Eq)]
struct MatchedPos {
    kind: MatchKind,
    pos: SingleLineSpan,
}

#[derive(Debug)]
pub struct Diff {
    pub(crate) lhs_src: FileContent,
    pub(crate) rhs_src: FileContent,
    pub(crate) hunks: Vec<Hunk>,

    lhs_positions: Vec<MatchedPos>,
    rhs_positions: Vec<MatchedPos>,

    /// If the two files do not have exactly the same bytes, the
    /// number of bytes in each file.
    pub(crate) has_byte_changes: Option<(usize, usize)>,
    pub(crate) has_syntactic_changes: bool,
}

/// Diff two trees, yielding one [`Diff`] per changed file (line-level [`Hunk`]s included).
///
/// Returns an iterator of `anyhow::Result<Diff>`: per-file diffing can fail independently, so
/// each item is fallible. The consumer drives it lazily and can stop at any point by dropping
/// the iterator (e.g. when the gRPC client hangs up).
pub fn diff<'repo>(
    repo: &'repo gix::Repository,
    old_tree: &gix::Tree<'_>,
    new_tree: &gix::Tree<'_>,
) -> anyhow::Result<DiffIter<'repo>> {
    let changes = repo.diff_tree_to_tree(Some(old_tree), Some(new_tree), None)?;
    let mut cache = repo.diff_resource_cache_for_tree_diff()?;
    cache.filter.options.large_file_threshold_bytes = LARGE_FILE_THRESHOLD_BYTES;
    Ok(DiffIter {
        repo,
        changes: changes.into_iter(),
        cache,
    })
}

/// Lazy iterator over per-file [`Diff`]s. Created by [`diff`].
pub struct DiffIter<'repo> {
    repo: &'repo gix::Repository,
    changes: std::vec::IntoIter<gix::object::tree::diff::ChangeDetached>,
    cache: gix::diff::blob::Platform,
}

impl Iterator for DiffIter<'_> {
    type Item = anyhow::Result<Diff>;

    fn next(&mut self) -> Option<Self::Item> {
        loop {
            let change = self.changes.next()?;
            let (old, new) = change_sides(&change);
            // Trees are recursed into by gix and never diffed directly; skip defensively.
            if old.is_kind(EntryKind::Tree) || new.is_kind(EntryKind::Tree) {
                continue;
            }
            // Submodules (gitlinks) can't go through the blob pipeline — their commit lives in
            // another repo. Emit the commit oids directly for the caller to compare.
            if old.is_kind(EntryKind::Commit) || new.is_kind(EntryKind::Commit) {
                return Some(Ok(submodule_diff(self.repo, &change, old, new)));
            }
            let result = build_diff(self.repo, &change, &mut self.cache);
            self.cache.clear_resource_cache_keep_allocation();
            return Some(result);
        }
    }
}

/// One side (old or new) of a change.
enum Side {
    Absent,
    Present { oid: ObjectId, kind: EntryKind },
}

impl Side {
    fn is_kind(&self, wanted: EntryKind) -> bool {
        matches!(self, Side::Present { kind, .. } if *kind == wanted)
    }
}

/// Decompose a change into its `(old, new)` sides regardless of variant.
fn change_sides(change: &gix::object::tree::diff::ChangeDetached) -> (Side, Side) {
    use gix::object::tree::diff::ChangeDetached as C;
    let present = |oid, mode: gix::object::tree::EntryMode| Side::Present {
        oid,
        kind: mode.kind(),
    };
    match change {
        C::Addition { id, entry_mode, .. } => (Side::Absent, present(*id, *entry_mode)),
        C::Deletion { id, entry_mode, .. } => (present(*id, *entry_mode), Side::Absent),
        C::Modification {
            previous_id,
            previous_entry_mode,
            id,
            entry_mode,
            ..
        } => (
            present(*previous_id, *previous_entry_mode),
            present(*id, *entry_mode),
        ),
        C::Rewrite {
            source_id,
            source_entry_mode,
            id,
            entry_mode,
            ..
        } => (
            present(*source_id, *source_entry_mode),
            present(*id, *entry_mode),
        ),
    }
}

/// Build a [`Diff`] for a submodule (gitlink) change: no hunks, both sides carry their commit
/// oid with `is_submodule` set. The caller compares the oids as submodule pointers.
fn submodule_diff(
    repo: &gix::Repository,
    change: &gix::object::tree::diff::ChangeDetached,
    old: Side,
    new: Side,
) -> Diff {
    let path = change.location().to_string();
    let side = |s: Side| FileContent {
        path: path.clone(),
        oid: match s {
            Side::Absent => ObjectId::null(repo.object_hash()).to_string(),
            Side::Present { oid, .. } => oid.to_string(),
        },
        size: 0,
        is_binary: false,
        is_submodule: true,
    };
    Diff {
        lhs_src: side(old),
        rhs_src: side(new),
        hunks: Vec::new(),
        lhs_positions: Vec::new(),
        rhs_positions: Vec::new(),
        has_byte_changes: None,
        has_syntactic_changes: false,
    }
}

/// Build the [`Diff`] for a single changed file.
fn build_diff(
    repo: &gix::Repository,
    change: &gix::object::tree::diff::ChangeDetached,
    cache: &mut gix::diff::blob::Platform,
) -> anyhow::Result<Diff> {
    use gix::diff::blob::platform::prepare_diff::Operation;
    use gix::diff::blob::platform::resource::Data;
    use gix::diff::blob::{Diff as BlobDiff, InternedInput};
    use gix::prelude::TreeDiffChangeExt as _;

    let change = change.attach(repo, repo);
    let blob = change.diff(cache)?;
    let out = blob.resource_cache.prepare_diff()?;

    // Per-side file metadata, derived from the prepared resources.
    let side = |res: &gix::diff::blob::platform::Resource<'_>| -> FileContent {
        let (size, is_binary) = match res.data {
            Data::Buffer { buf, .. } => (buf.len(), false),
            Data::Binary { size } => (size as usize, true),
            Data::Missing => (0, false),
        };
        FileContent {
            path: res.rela_path.to_string(),
            oid: res.id.to_string(),
            size,
            is_binary,
            is_submodule: false,
        }
    };
    let mut lhs_src = side(&out.old);
    let mut rhs_src = side(&out.new);

    let has_byte_changes = if lhs_src.size == rhs_src.size && out.old.id == out.new.id {
        None
    } else {
        Some((lhs_src.size, rhs_src.size))
    };

    let hunks = match out.operation {
        Operation::InternalDiff { algorithm } => {
            let input = InternedInput::new(out.old.intern_source(), out.new.intern_source());
            let mut d = BlobDiff::compute(algorithm, &input);
            d.postprocess_lines(&input);
            d.hunks()
                .map(|h| {
                    let before: Vec<LineNumber> = h.before.clone().map(LineNumber).collect();
                    let after: Vec<LineNumber> = h.after.clone().map(LineNumber).collect();
                    let n = before.len().max(after.len());
                    let lines = (0..n)
                        .map(|i| (before.get(i).copied(), after.get(i).copied()))
                        .collect();
                    Hunk {
                        novel_lhs: before.iter().copied().collect(),
                        novel_rhs: after.iter().copied().collect(),
                        lines,
                    }
                })
                .collect()
        }
        Operation::SourceOrDestinationIsBinary => {
            lhs_src.is_binary = true;
            rhs_src.is_binary = true;
            Vec::new()
        }
        Operation::ExternalCommand { .. } => unreachable!("we disabled that"),
    };

    Ok(Diff {
        lhs_src,
        rhs_src,
        hunks,
        lhs_positions: Vec::new(),
        rhs_positions: Vec::new(),
        has_byte_changes,
        has_syntactic_changes: false,
    })
}

/// Compute the two trees whose diff is the *interdiff* between an old patch
/// version (`from_base..from_head`) and a new one (`to_base..to_head`).
///
/// We rebase the old patch onto the new base so both versions share `to_base`,
/// then the caller diffs the returned trees (step 4):
///
/// ```text
/// from_base -- from_head        (old patch)
///     \
///      `-- to_base -- to_head   (new patch)
/// ```
///
/// 1. Rebase `from` onto `to_base` via a 3-way tree merge.
///    Replay the patch diff `from_base.tree -> from_head.tree` on top of `to_base.tree`.
/// 2. Return `(rebased_from_tree, to_head.tree)`, both rooted at `to_base`.
///
/// `from_base` / `to_base` default to the respective head commit's first parent.
///
/// `repo` is borrowed so the returned trees can reference it; resolving a
/// `repo_did` to an on-disk repository is out of scope here.
pub fn prepare_interdiff<'repo>(
    repo: &'repo gix::Repository,
    from: (Option<ObjectId>, ObjectId),
    to: (Option<ObjectId>, ObjectId),
) -> anyhow::Result<(gix::Tree<'repo>, gix::Tree<'repo>)> {
    let (from_base_id, from_head_id) = from;
    let (to_base_id, to_head_id) = to;

    let from_head = repo.find_commit(from_head_id)?;
    let to_head = repo.find_commit(to_head_id)?;

    let from_base_id = from_base_id.unwrap_or_else(|| resolve_base(&from_head).unwrap());
    let to_base_id = to_base_id.unwrap_or_else(|| resolve_base(&to_head).unwrap());

    let from_base = repo.find_commit(from_base_id)?;
    let to_base = repo.find_commit(to_base_id)?;

    // Endpoint trees for the 3-way merge.
    let from_base_tree = from_base.tree_id()?.detach();
    let from_head_tree = from_head.tree_id()?.detach();
    let to_base_tree = to_base.tree_id()?.detach();

    // Rebase = replay (from_base_tree -> from_head_tree) onto to_base_tree:
    //   ancestor = from_base_tree, ours = to_base_tree, theirs = from_head_tree.
    let options = repo.tree_merge_options()?;
    let labels = gix::merge::blob::builtin_driver::text::Labels {
        ancestor: Some("from.base".as_bytes().as_bstr()),
        current: Some("to.base".as_bytes().as_bstr()),
        other: Some("from.head".as_bytes().as_bstr()),
    };
    let mut outcome = repo.merge_trees(
        from_base_tree,
        to_base_tree,
        from_head_tree,
        labels,
        options,
    )?;

    // Interdiff tolerates conflicts: the merged tree carries conflict markers
    // (blob driver keeps them); surface them rather than failing.
    if !outcome.conflicts.is_empty() {
        info!(
            conflicts = outcome.conflicts.len(),
            "interdiff rebase produced conflicts; tree contains conflict markers"
        );
    }

    let rebased_id = outcome.tree.write()?.detach();
    let rebased_tree = repo.find_tree(rebased_id)?;
    let to_head_tree = to_head.tree()?;
    Ok((rebased_tree, to_head_tree))
}

fn resolve_base(commit: &Commit) -> anyhow::Result<ObjectId> {
    commit
        .parent_ids()
        .next()
        .map(|id| id.detach())
        .ok_or_else(|| anyhow::anyhow!("commit {0} has no parent to use as base", commit.id))
}

#[cfg(test)]
mod tests {
    use super::*;

    /// A fresh bare repo in a tempdir. The `TempDir` is returned so it outlives the repo.
    fn repo() -> (tempfile::TempDir, gix::Repository) {
        let dir = tempfile::tempdir().unwrap();
        let repo = gix::init_bare(dir.path()).unwrap();
        (dir, repo)
    }

    /// Build a tree from `(path, content)` pairs and return its oid.
    fn tree(repo: &gix::Repository, files: &[(&str, &[u8])]) -> ObjectId {
        let empty = repo.empty_tree().id;
        let mut editor = repo.edit_tree(empty).unwrap();
        for (path, content) in files {
            let blob = repo.write_blob(content).unwrap().detach();
            editor.upsert(*path, EntryKind::Blob, blob).unwrap();
        }
        editor.write().unwrap().detach()
    }

    /// Build a tree with a single submodule (gitlink) entry at `path` pointing at `commit_oid`.
    fn tree_with_gitlink(repo: &gix::Repository, path: &str, commit_oid: ObjectId) -> ObjectId {
        let empty = repo.empty_tree().id;
        let mut editor = repo.edit_tree(empty).unwrap();
        editor.upsert(path, EntryKind::Commit, commit_oid).unwrap();
        editor.write().unwrap().detach()
    }

    /// Commit `tree_oid` with the given parents. Uses an explicit signature because a fresh
    /// `init_bare` repo has no user config (config-reading `commit` would fail AuthorMissing).
    fn commit(repo: &gix::Repository, tree_oid: ObjectId, parents: &[ObjectId]) -> ObjectId {
        let sig = gix::actor::SignatureRef {
            name: b"t".as_bstr(),
            email: b"t@t".as_bstr(),
            time: "0 +0000",
        };
        repo.new_commit_as(sig, sig, "msg", tree_oid, parents.iter().copied())
            .unwrap()
            .id
    }

    /// Collect all diffs between two tree oids.
    fn diffs(repo: &gix::Repository, old: ObjectId, new: ObjectId) -> Vec<Diff> {
        let old = repo.find_tree(old).unwrap();
        let new = repo.find_tree(new).unwrap();
        diff(repo, &old, &new)
            .unwrap()
            .collect::<anyhow::Result<Vec<_>>>()
            .unwrap()
    }

    #[test]
    fn modification() {
        let (_dir, repo) = repo();
        let old = tree(&repo, &[("a", b"x\n")]);
        let new = tree(&repo, &[("a", b"y\n")]);

        let got = diffs(&repo, old, new);
        assert_eq!(got.len(), 1);
        let d = &got[0];
        assert_eq!(d.lhs_src.path, "a");
        assert_eq!(d.rhs_src.path, "a");
        assert!(!d.lhs_src.is_binary && !d.rhs_src.is_binary);
        assert_eq!(d.has_byte_changes, Some((2, 2)));
        assert_eq!(d.hunks.len(), 1);
        let h = &d.hunks[0];
        assert!(h.novel_lhs.contains(&LineNumber(0)));
        assert!(h.novel_rhs.contains(&LineNumber(0)));
    }

    #[test]
    fn addition() {
        let (_dir, repo) = repo();
        let old = tree(&repo, &[]);
        let new = tree(&repo, &[("a", b"hello\n")]);

        let got = diffs(&repo, old, new);
        assert_eq!(got.len(), 1);
        let d = &got[0];
        assert_eq!(d.lhs_src.size, 0); // old side missing
        assert_eq!(d.rhs_src.size, 6);
        assert_eq!(d.rhs_src.path, "a");
        assert!(d.has_byte_changes.is_some());
    }

    #[test]
    fn deletion() {
        let (_dir, repo) = repo();
        let old = tree(&repo, &[("a", b"hello\n")]);
        let new = tree(&repo, &[]);

        let got = diffs(&repo, old, new);
        assert_eq!(got.len(), 1);
        let d = &got[0];
        assert_eq!(d.lhs_src.size, 6);
        assert_eq!(d.rhs_src.size, 0); // new side missing
        assert_eq!(d.lhs_src.path, "a");
    }

    #[test]
    fn binary() {
        let (_dir, repo) = repo();
        let old = tree(&repo, &[("a", b"\x00\x01bin\n")]);
        let new = tree(&repo, &[("a", b"\x00\x01BIN\n")]);

        let got = diffs(&repo, old, new);
        assert_eq!(got.len(), 1);
        let d = &got[0];
        assert!(d.lhs_src.is_binary && d.rhs_src.is_binary);
        assert!(d.hunks.is_empty());
    }

    #[test]
    fn multiple_files_and_laziness() {
        let (_dir, repo) = repo();
        let old = tree(&repo, &[("a", b"1\n"), ("b", b"1\n")]);
        let new = tree(&repo, &[("a", b"2\n"), ("b", b"2\n")]);

        let old_t = repo.find_tree(old).unwrap();
        let new_t = repo.find_tree(new).unwrap();

        // Lazy: pulling one item yields exactly one without consuming the rest.
        let mut it = diff(&repo, &old_t, &new_t).unwrap();
        assert!(it.next().is_some());

        let all = diffs(&repo, old, new);
        assert_eq!(all.len(), 2);
        let mut paths: Vec<_> = all.iter().map(|d| d.rhs_src.path.clone()).collect();
        paths.sort();
        assert_eq!(paths, vec!["a".to_string(), "b".to_string()]);
    }

    #[test]
    fn directory_addition_yields_inner_blobs() {
        let (_dir, repo) = repo();
        let old = tree(&repo, &[]);
        // A whole new directory: gix emits a Tree change for `dir/` plus blob changes for the
        // files inside. We must skip the tree and yield only the blobs (no error).
        let new = tree(&repo, &[("dir/a", b"x\n"), ("dir/b", b"y\n")]);

        let got = diffs(&repo, old, new);
        let mut paths: Vec<_> = got.iter().map(|d| d.rhs_src.path.clone()).collect();
        paths.sort();
        assert_eq!(paths, vec!["dir/a".to_string(), "dir/b".to_string()]);
    }

    #[test]
    fn interdiff_equal_patches_is_empty() {
        let (_dir, repo) = repo();
        // Shared base content B; patch P turns a's "1\n" into "2\n".
        let base = commit(&repo, tree(&repo, &[("a", b"1\n")]), &[]);
        let patched = tree(&repo, &[("a", b"2\n")]);
        // `from` and `to` both apply the same patch P on top of the same base.
        let from_head = commit(&repo, patched, &[base]);
        let to_head = commit(&repo, patched, &[base]);

        let (rebased, to_head_tree) =
            prepare_interdiff(&repo, (Some(base), from_head), (Some(base), to_head)).unwrap();

        // Same patch ⇒ interdiff (diff of rebased vs to_head) is empty.
        let got = diff(&repo, &rebased, &to_head_tree)
            .unwrap()
            .collect::<anyhow::Result<Vec<_>>>()
            .unwrap();
        assert!(got.is_empty(), "expected empty interdiff, got {got:?}");
    }

    #[test]
    fn interdiff_divergent_shows_delta() {
        let (_dir, repo) = repo();
        let base = commit(&repo, tree(&repo, &[("a", b"1\n")]), &[]);
        // `from` applies patch P (a: 1->2).
        let from_head = commit(&repo, tree(&repo, &[("a", b"2\n")]), &[base]);
        // `to` applies P plus a new file `c`.
        let to_head = commit(&repo, tree(&repo, &[("a", b"2\n"), ("c", b"new\n")]), &[base]);

        let (rebased, to_head_tree) =
            prepare_interdiff(&repo, (Some(base), from_head), (Some(base), to_head)).unwrap();

        let got = diff(&repo, &rebased, &to_head_tree)
            .unwrap()
            .collect::<anyhow::Result<Vec<_>>>()
            .unwrap();
        assert_eq!(got.len(), 1);
        assert_eq!(got[0].rhs_src.path, "c");
    }

    #[test]
    fn submodule_modification() {
        let (_dir, repo) = repo();
        // Two distinct commit oids for the gitlink to point at (need not be real submodules).
        let a = commit(&repo, tree(&repo, &[("x", b"1\n")]), &[]);
        let b = commit(&repo, tree(&repo, &[("x", b"2\n")]), &[]);
        let old = tree_with_gitlink(&repo, "sub", a);
        let new = tree_with_gitlink(&repo, "sub", b);

        let got = diffs(&repo, old, new);
        assert_eq!(got.len(), 1);
        let d = &got[0];
        assert!(d.lhs_src.is_submodule && d.rhs_src.is_submodule);
        assert!(d.hunks.is_empty());
        assert_eq!(d.lhs_src.path, "sub");
        assert_eq!(d.lhs_src.oid, a.to_string());
        assert_eq!(d.rhs_src.oid, b.to_string());
    }

    #[test]
    fn submodule_addition() {
        let (_dir, repo) = repo();
        let b = commit(&repo, tree(&repo, &[("x", b"1\n")]), &[]);
        let old = tree(&repo, &[]);
        let new = tree_with_gitlink(&repo, "sub", b);

        let got = diffs(&repo, old, new);
        assert_eq!(got.len(), 1);
        let d = &got[0];
        assert!(d.rhs_src.is_submodule);
        assert!(d.hunks.is_empty());
        assert_eq!(d.rhs_src.oid, b.to_string());
        // Absent old side reported as the null oid.
        assert_eq!(d.lhs_src.oid, ObjectId::null(repo.object_hash()).to_string());
    }
}